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EPA EcoBox

EPA EcoBox Tools - Exposure Factors


Exposure FactorsTerrestrial receptors can be exposed to environmental contaminants through inhalation, dermal contact with contaminated water or soil, or ingestion of contaminated food, water, or soil. Aquatic receptors can be exposed through direct absorption from the water column, ingestion of contaminated food and water, uptake from sediment and sediment pore waters, and incidental ingestion of sediment. Exposure factorsHelpexposure factors Factors related to human behavior and characteristics that help determine an individual's exposure to an agent. are generally more likely to be available for terrestrial receptors.

For implementation of an exposure assessment to ecological receptors, species-specific exposure parameter values can be used (e.g., food, water, and soil ingestion rates; frequency and duration of exposure; home range/territory size). For ecological risk assessments, EPA’s Wildlife Exposure Factors Handbook (U.S. EPA, 1993) and Sample et al. (1997) are principal sources of wildlife exposure factor data. More recently, EPA developed the Wildlife Scenario Builder (WSB) to help risk assessors estimate air, water, and dietary intake rates for a variety of North American Wildlife species (U.S. EPA, 2013).

Wildlife Exposure Factors Handbook (Wildlife EFH)—EPA, 1993

EPA’s Wildlife Exposure Factors Handbook (Wildlife EFH) provides information on various factors used to assess exposure to wildlife. These values are based on a 1990 literature search, supplemented by targeted searches conducted in 1992. Volume I summarizes literature values for exposure factors for 34 species of birds, mammals, amphibians, and reptiles. Common equations used to estimate exposure to wildlife receptors are also described in Volume I. Volume II is the literature review database that accompanies the selected species profiles provided in Chapter 2 of Volume I. The Wildlife EFH provides both guidance and data to facilitate estimating wildlife exposure to contaminants in the environment (U.S. EPA, 1993).

The Wildlife EFH focuses on selected groups of mammals, birds, amphibians, and reptiles that include the following (U.S. EPA, 1993):

  • Mammals—short-tailed shrew, red fox, raccoon, mink, river otter, harbor seal, deer mouse, prairie vole, meadow vole, muskrat, eastern cottontail
  • Birds—great blue heron, Canada goose, mallard, lesser scaup, osprey, red-tailed hawk, bald eagle, American kestrel, northern bobwhite, American woodcock, spotted sandpiper, herring gull, belted kingfisher, marsh wren, American robin
  • Amphibians and reptiles—snapping turtle, painted turtle, eastern box turtle, racer, northern water snake, eastern newt, green frog, bullfrog

Fish and aquatic or terrestrial invertebrates are not included in Wildlife EFH. Profiles on amphibians and reptiles are, in general, less developed than those for birds and mammals. Methods for assessing exposure to birds and mammals are more common and well developed (U.S. EPA, 1993).

Chapter 2 of the Wildlife EFH presents exposure profiles for the selected species along with brief descriptions of their natural history. Each species profile includes an introduction to the species' general taxonomic group, qualitative description of the species, list of similar species, and table of exposure factors. Wildlife exposure factors in the Wildlife EFH include:

  • Normalizing factors (body weight, growth rate, metabolic rate);
  • Contact rates (food ingestion rate, dietary composition, water ingestion rate, soil/sediment intake rate, inhalation rate, dermal surface area);
  • Population dynamics (social organization, home range size, population density, annual fecundity, age at sexual maturity, annual mortality rates, average longevity); and
  • Seasonal activities (mating season, parturition/hatching, molt/metamorphosis, dispersal/migration/hibernation).

See the table below for additional details on the receptor-group-specific exposure parameter values presented in Chapter 2 of the Wildlife EFH Volume I.

Exposure Factor Type Receptor Group Exposure Factors Included (If Available)
Normalizing and Contact Rate Factors All animals Body weight; metabolic rate; food ingestion rate; water ingestion rate; sediment/soil ingestion rate; inhalation rate; surface area (dermal)
Birds Egg weight;  weight at hatching; chick or nestling growth rate; weight at fledging
Mammals Neonate weight; pup growth rate; weight at weaning
Reptiles and amphibians Body length; egg weight; weight at hatching; juvenile growth rate; tadpole weight (frogs only); larval or eft weight; (newts only)
Dietary Composition All animals Dietary composition by season
Population Dynamics All animals Home range size/territory size/foraging radius; population density; age at sexual maturity; annual mortality rates; longevity
Birds Clutch size; clutches per year; days incubation; age at fledging; number fledged per active nest; percent nests successful; number fledged per successful nest
Mammals Litter size; litters per year; days gestation; pup growth rate; age at weaning
Reptiles and amphibians Clutch or litter size; clutches or litters per year; days incubation; juvenile growth rate; length at sexual maturity
Seasonal Activities All animals Begin Month, peak month(s), end month for each activity below
Birds Mating and laying
Mammals Mating and parturition
Reptiles and amphibians Mating and nesting

Chapter 3 of the Wildlife EFH Volume I provides allometric equations that can be used to estimate exposure parameter values—on the basis of animal body weight— for species for which measured values are not available. Empirically-derived allometric equations are provided that relate food ingestion rates, water intake rates, inhalation rates, surface area, and metabolic rate to body weight (U.S. EPA, 1993). The taxonomic relationship between the known species and the one of interest is important to understand. The closer they are related—considering uptake, metabolism, depuration of a chemical—the more likely toxic responses will be similar (U.S. EPA, 1998).

Chapter 4 in Volume I describes common equations used to estimate wildlife exposure to environmental contaminants. Included are methods for estimating diet-specific food ingestion rates on the basis of metabolic rate and for estimating exposure to contaminants in soil and sediment.

The Wildlife EFH might be used as a framework to guide development of exposure factors and assessments for species of concern in a risk assessment. Species selection criteria for site-specific risk assessments will vary and might consider the following: species that play important roles in community structure or function (e.g., top predators or major herbivores); diet, habitat preferences, and behaviors that make the species likely to contact the stressor; species from different taxa that might exhibit different toxic effects from stressors; and local species that are of concern to Federal and state regulatory agencies (e.g., endangered and threatened species) (U.S. EPA, 1993).

Methods and Tools for Estimation of the Exposure of Terrestrial Wildlife to Contaminants—Sample et al., 1997

Methods and Tools for Estimation of the Exposure of Terrestrial Wildlife to Contaminants (1997) (120 pp, 561 K, About PDF) provides life history parameter values for use in exposure assessment for eight mammals and five birds (selected based on their likely occurrence at U.S. Department of Energy facilities). Parameters described include distribution (range), body weight/size, food habits and diet composition, food and water consumption rates, soil ingestion, respiration rate, metabolism, habitat requirements, home range, population density, population dynamics/survival, reproduction/breeding, and other behavior.

Wildlife Scenario Builder (WSB)—U.S. EPA, 2013

The Wildlife Scenario Builder (WSB) includes wildlife and life history databases with information on 49 North American species of wildlife, including 24 species of birds, 17 species of mammals, 3 species of amphibians, and 5 species of reptiles (see Table 1). The life history database contains natural history information (e.g., distribution, habitat, diet) and physiological data (body weight, intake requirements for air, water and food/energy) relevant to wildlife exposure assessment. The information in the database was extracted primarily from EPA’s Wildlife EFH (U.S. EPA, 1993).The WSB makes calculating exposure via the diet easier and more accurate by calculating energy requirements using allometric equations; calculating the energy content of food items; linking food energy content with measured dietary item intakes; and allowing for user adjustment of diet scenarios.

Table 1. Species Included in the WSB
Birds Mammals Amphibians and Reptiles
American kestrel
American robin
American woodcock
Bald eagle
Bay-breasted warbler
Belted kingfisher
Black scoter
Canada goose
Common loon
Great blue heron
Great horned owl
Herring gull
Lesser/greater scaup
Marsh wren
Northern bobwhite
Red-tailed hawk
Red-winged blackbird
Ruffed grouse
Savannah sparrow
Spotted sandpiper
Tree swallow
Yellow warbler
Deer mouse
Eastern cottontail
Harbor seal
Little brown bat
Masked shrew
Meadow vole
Prairie vole
Short-tailed shrew
White-tailed deer
Wolf species (timber/gray)
Eastern newt
Green frog
Eastern box turtle
Northern water snake
Painted turtle
Snapping turtle

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